Split-ring seals are useful in many applications to seal an annular space between a cylindrical member fitted in a bore. Conventional piston rings used in engines are a well-known example of such seals. Each ring is expanded to slide over the piston into an annular groove. The ring is then compressed into the groove when the piston is fitted into a cylinder. The resiliency of the ring urges the ring periphery against the cylinder wall to effect the desired seal.
Known styles of seal rings are not always satisfactory in applications involving severe vibratory motion because the ring wears rapidly, and the loading on the ring may decrease rapidly with progressive wear until the sealing effect is lost. Backup springs have been used in the ring groove between the piston (or equivalent part) and inner ring surface, but a conventional high-rate spring is of limited assistance in compensating for ring wear because the restoring force acting on the ring to effect a seal drops rapidly with a slight decrease in ring cross-section due to wear.
The problem of seal-ring wear is aggravated in applications where extensive dismantling of an assembly or machine is necessary to replace the ring. For example, aircraft fan-jet engines have a severe sealing problem in bleed-air ducts used to convey hot gas from a central compressor section radially outwardly through an outer housing or fan case to pipes leading to wing deicers or other accessories.
Large and variable temperature differentials exist between the compressor-section case and fan case during the various operating modes of the engine, and considerable longitudinal and radial freedom must be built into the bleed-air duct to compensate for dimensional changes of the compressor section and fan case arising from these temperature variations. That is, the duct must be able to elongate or contract to compensate for changes in the annular space between the compressor and fan cases as these parts radially expand or contract relative to each other.
To provide the necessary longitudinal and radial freedom, the bleed-air duct between the compressor and fan cases is typically formed by a loosely fitted pair of telescoped tubes which are longitudinally movable to vary the length of the duct during engine operation. The tubes are sealed (to prevent loss of the high-pressure bleed-air) by a conventional seal-ring (similar to a piston ring) fitted between the inner and outer tubes. The operating temperature of the duct is usually too high to permit lubrication of the seal ring, so the telescoped tubes in effect form a dry (unlubricated) slip joint.
Conventional seal rings tend to wear rapidly due to the high-vibration and high-temperature environment of the bleed duct. After a little initial wear has occurred, the interface load of the seal ring drops sharply, leading to more rapid wear as the parts rattle and impact each other with increasing force. The result is that relatively frequent replacement of the seal ring and duct tubes is required, and an expensive dismantling of the engine is involved in doing this work.
The seal ring of this invention provides a long-life seal for bleed ducts, and substantially extends the time-between-overhaul period for these parts. Although the new ring is somewhat more expensive to make than a conventional piston ring, this cost increase is negligible when compared to the expense of engine teardown and aircraft out-of-service time caused by use of known seals. Use of the new seal ring is not restricted to jet-engine bleed ducts, and the ring has utility in many other applications requiring minimum wear of the sealed parts, and where a relatively constant or "flat" seal loading is desired in spite of conditions which produce relatively rapid wear of the ring.